GB2094115A - Convergence adjustment in colour television - Google Patents

Abstract

To adjust convergence in a colour television display device, a circuit for driving a correction current through a convergence coil (18) comprises means (16,20) to sample and hold the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area and means (22) for varying the d.c. component of the current flowing through the convergence coil (18) in order to cancel out any d.c. component present in the coils at the centre of the picture area. The dynamic correction current then does not disturb the previous static convergence adjustment (not shown) at the screen centre. <IMAGE>

Description

SPECIFICATION Convergence adjustment in colour television The present invention relates to the adjustment of convergence in a colour television display device.

In a colour television display device such as a shadow mask cathode ray tube, adjustment of convergence or the coincidence of the three electron beams to produce a registered picture, is important.

This is usually achieved by passing a variety of waveforms derived from line and field sawtooth and/or parabolas through sets of coils placed in close proximity to each of the three guns.

The required drive for the coils can be split into two parts namely the static convergence current and the dynamic convergence current.

The term "static convergence" relates to the convergence of the three beams near the centre of the screen area. This can be likened to a shift of each of the rashers to achieve coincidence at the centre.

There may still be misconvergence at the corners and edges of the picture, however. Correction for the latter misconvergence, known as "dynamic correction" is provided by the a.c. component of the waveforms.

When adjusting the convergence, the usual procedure is to set the static convergence first such that the centre of the picture area is in register. The dynamic convergence controls are then adjusted to produce correct convergence at the edges and the corners of the picture. It is undesirable for these dynamic controls to affect the centre of the picture but this has happened in the past due to the difficulty of ensuring that there is zero current flowing in the coils at the centre of the picture.

The present invention is concerned with the mitigation of the foregoing disadvantage.

According to the present invention, there is provided a circuit for driving a correction current through a convergence coil which comprises means for sensing the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area and means for varying the d.c. component of the current flowing through the convergence coil in order to cancel out any d.c.

component present in the coils at the centre of the picture area.

The invention will now be described further by way of example with reference to the accompanying drawing with is a block circuit diagram of a circuit in accordance with the invention.

A convergence coil driver 10 receives a dynamic correction waveform through a first input 12 and a d.c. correction signal 14 serving to set the static convergence. A resistor 16 is connected in series with the convergence coil 18 so that a voltage is developed across the resistor 16 proportional to the current flowing through the convergence coil 18.

This voltage is applied to a sample and hold circuit which is gated by a signal corresponding to the instant when the beams are pointing at the centre of the screen. The value of the convergence current at the centre of the screen is thus sampled and held and is compared in a differential amplifier 22 with a signal of reference earth potential to provide an error signal fed back to the d.c. input of the convergence coil driver 10.

CLAIMS (Filed on 2 March 1982) 1. A circuit for driving a correction current through a convergence coil which comprises means for sensing the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area and means for varying the d.c. component of the current flowing through the convergence coil in order to cancel out any d.c.

component present in the coils at the centre of the picture area.

2. Acircuitas claimed in claim 1, wherein the means for sensing the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area comprises a sample and hold circuit which is gated by a signal corresponding to the instant when the beam is pointing at the centre of the picture area.

3. A circuit as claimed in claim 1 or 2, wherein the means for varying the d.c. component of the current flowing through the convergence coil comprises a differential amplifier which compares a signal corresponding to the instant when the beam is pointing at the centre of the picture with a reference signal so as to provide an error signal which varies the d.c.

component of the current flowing through the convergence coil.

4. A circuit as claimed in claims 2 and 3, wherein a resistor is connected in series with the convergence coil, voltage developed across the resistor being applied to the sample and hold circuit such that the value of the convergence currents at the centre of the picture is compared in the differential amplifier with a signal of reference earth potential so as to provide the error signal.

5. A colour television comprising a circuit as claimed in any preceding claim.

6. A circuit for driving a correction current through a convergence coil substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Convergence adjustment in colour television The present invention relates to the adjustment of convergence in a colour television display device. In a colour television display device such as a shadow mask cathode ray tube, adjustment of convergence or the coincidence of the three electron beams to produce a registered picture, is important. This is usually achieved by passing a variety of waveforms derived from line and field sawtooth and/or parabolas through sets of coils placed in close proximity to each of the three guns. The required drive for the coils can be split into two parts namely the static convergence current and the dynamic convergence current. The term "static convergence" relates to the convergence of the three beams near the centre of the screen area. This can be likened to a shift of each of the rashers to achieve coincidence at the centre. There may still be misconvergence at the corners and edges of the picture, however. Correction for the latter misconvergence, known as "dynamic correction" is provided by the a.c. component of the waveforms. When adjusting the convergence, the usual procedure is to set the static convergence first such that the centre of the picture area is in register. The dynamic convergence controls are then adjusted to produce correct convergence at the edges and the corners of the picture. It is undesirable for these dynamic controls to affect the centre of the picture but this has happened in the past due to the difficulty of ensuring that there is zero current flowing in the coils at the centre of the picture. The present invention is concerned with the mitigation of the foregoing disadvantage. According to the present invention, there is provided a circuit for driving a correction current through a convergence coil which comprises means for sensing the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area and means for varying the d.c. component of the current flowing through the convergence coil in order to cancel out any d.c. component present in the coils at the centre of the picture area. The invention will now be described further by way of example with reference to the accompanying drawing with is a block circuit diagram of a circuit in accordance with the invention. A convergence coil driver 10 receives a dynamic correction waveform through a first input 12 and a d.c. correction signal 14 serving to set the static convergence. A resistor 16 is connected in series with the convergence coil 18 so that a voltage is developed across the resistor 16 proportional to the current flowing through the convergence coil 18. This voltage is applied to a sample and hold circuit which is gated by a signal corresponding to the instant when the beams are pointing at the centre of the screen. The value of the convergence current at the centre of the screen is thus sampled and held and is compared in a differential amplifier 22 with a signal of reference earth potential to provide an error signal fed back to the d.c. input of the convergence coil driver 10. CLAIMS (Filed on 2 March 1982)

1. A circuit for driving a correction current through a convergence coil which comprises means for sensing the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area and means for varying the d.c. component of the current flowing through the convergence coil in order to cancel out any d.c.

component present in the coils at the centre of the picture area.

2. Acircuitas claimed in claim 1, wherein the means for sensing the value of the convergence current at an instant corresponding to the beam lying in the centre of the picture area comprises a sample and hold circuit which is gated by a signal corresponding to the instant when the beam is pointing at the centre of the picture area.

3. A circuit as claimed in claim 1 or 2, wherein the means for varying the d.c. component of the current flowing through the convergence coil comprises a differential amplifier which compares a signal corresponding to the instant when the beam is pointing at the centre of the picture with a reference signal so as to provide an error signal which varies the d.c.

component of the current flowing through the convergence coil.

4. A circuit as claimed in claims 2 and 3, wherein a resistor is connected in series with the convergence coil, voltage developed across the resistor being applied to the sample and hold circuit such that the value of the convergence currents at the centre of the picture is compared in the differential amplifier with a signal of reference earth potential so as to provide the error signal.

5. A colour television comprising a circuit as claimed in any preceding claim.

6. A circuit for driving a correction current through a convergence coil substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.